The invention relates to a method for interferometric radar measurements.
Due to their construction, radar devices are precise range-finding systems, which means that without special measures, a radar device is capable of determining only the distance of a target from the antenna, but not its direction. It is possible to determine only whether a target is present within the lobe of the antenna.
This problem is eliminated to a large extent in conjunction with the ROSAR or Heli-Radar system known until now by using, for example 16 vertically staggered antennas with an antenna opening angle of, for example 2.5°. It is possible with this type of a system to determine the location of an elevated obstruction etc. within an accuracy of about 2.5° in terms of elevation. However, in this case, targets located at the same distance are also displayed in the same antenna in the same image spot.
The azimuthal resolution of the known Heli-Radar system amounts to about 0.2° because of a special signal processing. These features are taught in the disclosure in DE 39 22 086 C1. However, the direction of an obstruction and thus the location in space at which this obstruction is located can be determined only with the help of a triangulation, whereby in the simplest case, two locally separated radar installations can be used for this purpose.
However, it is also possible to use the properties of a coherent radar system and to perform a triangulation with the help of the phase of the emitted signal. Thus, a coherent radar system is used which coherently transmits a signal via a transmitting antenna and receives the echoes scattered back via two locally separated receiving antennas. A coherent evaluation permits a calculation of the phase difference between receiving signals. The direction from which the scattered echoes are received is determined based on the phase difference. Now, once the distance and direction of an “obstruction” have been computed, its location in space can be determined as well. This type of three-dimensional determination of a location with the help of a coherent radar system which comprises one transmitting antenna and two receiving antennas is generally referred to as “radar interferometry” and known for a long time. It is used already for the generation of topographic charts with the help of SAR-systems installed on aircraft, for example by the DOSAR system of the firm Dornier GmbH.
With these designs, reference is also made to the following published documents pertaining to the further state of the art:
C. T. Allan, Review Article, Interferometric Synthetic Aperture Radar, in IEEE Geoscience and Remote Sensing Society News Letter, September 1995, p. 6 ff; S. Buckreuss, J. Moreira, H. Rinkel and G. Waller, Advanced SAR Interferometry Study, DLR Bulletin 94, Jun. 10, 1994, Institut für Hochfrequenztechnik, Oberpfaffenhofen.
The entire prior art known to this date and the state of the art cited above, including the ROSAR principle on which the present invention is based, projects terrain elevations or other elevated obstructions in one plane, so that it is not possible to recognize the elevation of the given obstruction if the reproduced topography of the terrain present is unknown. However, a three-dimensional image is required for controlling flights.
The present invention is based on the problem of proposing measures on the basis of the ROSAR principle that permit a quasi-three-dimensional image representation of terrain and other obstructions.